The present invention is directed to a temperature responsive fluid coupling device of the type having a fluid medium for transmitting torque between relatively rotatable input and output coupling members and more specifically to a sheer type fluid coupling device wherein the amount of the fluid medium transmitting torque between the rotatable members can be varied in response to changes in temperature by means of a bimetal spiral spring to thereby vary the speed of the output member. Fluid couplings of this type are used in driving different kinds of load devices and are particularly used for driving a cooling fan device for an internal combustion engine.
A conventional fluid coupling device of this type is disclosed, for example, in Japanese utility model laid open publication No. 55-115445. FIG. 1 of the present application shows the basic construction of this conventional fluid coupling device 10 which includes an input coupling member 11 adapted to be driven by an engine (not shown) and an output coupling member 12 to which a cooling fan (not shown) may be secured. The output coupling member 12 is rotatably mounted on the input coupling member 11 by a suitable bearing 13, disposed therebetween. A rotor 14 is secured to the coupling member 11 in the form of a shaft by suitable spline means. The output coupling member 12 is in the form of a housing having a hollow interior which is divided into a working chamber 16 and a reservior chamber 17 by means of a partition plate 15 in the form of a disc. The rotor 14 is disposed in the working chamber 16 and a viscous fluid, such as a silicon oil, is circulated between the reservoir chamber and the working chamber. Adjacent surface portions of the rotor 14 and the output coupling member 12 are provided with torque transmitting elements comprised of a pluralitly of cooperating grooves and projections which generally form a labyrinth L. The partition plate 15 is provided at the outer circumference portion thereof with a fluid return passage or hole 18 through which the viscous fluid is permitted to flow from the reservoir chamber 17 to the working chamber 16. The output coupling member 12 is provided with a pumping passage or hole 19 through which the viscous fluid can be transferred from the working chamber 16 to the reservior chamber 17 by means of a discharge force generated by a pumping projection P. A rod or stub shaft 20 is rotatably supported concentrically with the input and output coupling members 11 and 12. A valve plate 21 is connected to one end of the shaft 20 and is positioned against the partition plate 15 and a bimetal spiral spring 22 is connected to the other end of the shaft 20 on the front side surface of the output coupling member 12. The valve plate 21 will be rotated on the partition plate 15 by the expansion and contraction of the bimetal spring 22 which is subjected to changes in the temperature of the radiator air. Therefore, the fluid return passage 18 can be controlled to be opened and closed by the valve plate 21, thereby controlling the flow of viscous fluid from the reservoir chamber 17 to the working chamber 16.
The shaft 20 is rotatably supported by a bushing 23 which in turn is supported by the front side wall 25 of the output coupling member 12. A pair of O-rings 24 of synthetic rubber material are disposed between the shaft 20 and the bushing 23 so as to prevent the escape of fluid from the reservoir chamber 17 by leaking along and around the shaft 20. However, during the rotation or axial sliding movement of the shaft 20, a reaction is produced by friction to increase rotating and the sliding torque of the rod 20.
When the engine temperature is low, the fluid return passage 18 is closed by the valve plate 21 to thereby prevent the fluid flow from the reservoir chamber 17 to the working chamber 16. At this time the fluid can be transferred from the working chamber 16 to the reservoir chamber 17 by means of the discharge action of the pumping projection P. Therefore, due to the small quantity of fluid remaining in the working chamber 16, the rotational speed of the output coupling member 12 is maintained at a very low level whereby the cooling fan is driven at a slow speed. Under such operating conditions the coupling is considered to be in the OFF condition. When the engine temperature rises and reaches a predetermined value, the bimetal spring 22 will actuate the valve plate 21 through the shaft 20 to thereby open the fluid passage 18, permitting the fluid to flow from the reservoir chamber 17 to the working chamber 16. Therefore, due to an increase in the amount of fluid in the working chamber 16, the rotational speed of the output coupling member 12 will increase to drive the cooling fan at a faster speed. Under such an operating condition, the coupling is considered to be in the ON condition.
Since the shaft 20 is rotatably supported by the bushing through the O-rings 24, the O-rings 24 will slide on the bushing 23 and a sliding torque is produced between the O-rings 24 and the bushing 23. Therefore, due to the sliding torque, the response to temperature changes decreases when the fluid coupling device changes from the OFF condition to the ON condition when the temperature rises to thereby increase the thermal hysteresis of the coupling device which shows the difference between the temperature when the coupling device is changed from the OFF condition to the ON condition on the rise of the temperature, and the temperature when the coupling device is changed from the ON condition to the OFF condition on the fall of the temperature.
In order to decrease the sliding torque produced by the rotation of the shaft 20, it has been suggested to support the shaft 20 on the front side wall of the output coupling member 12 through a sealed bearing. According to such an arrangement, the sealed bearing would be mounted on the front side wall with the seal plate of the bearing exposed to the outside. As a result, the seal plate, especially the lip seal portion, will be susceptible to deterioration by moisture and salts which may enter the engine compartment through the front grill and the radiator core fin. Since the moisture and the salts may enter into the bearing, the interior of the bearing will gather rust, especially when the car is stopped for a long period of time. Since the coupling device is maintained in OFF condition when the car is stopped, the rusting of the bearing will cause the bearing to lock up and prevent the valve plate from rotating on the partition plate when the engine is subsequently started. As a result, the coupling device can not be shifted to the ON condition, thereby placing the engine in danger of overheating.